Introducing a New Product and Technology

Developing an Energy-saving Rudder with Bulb Fins
~ Achieving both Steering Effectiveness and Energy-saving ~Yoshikazu Tanaka, Isao Yoshino, Akira Taniguchi - R&D Promotion Office
MOL Techno-Trade Co., Ltd.

1. Introductionenergy-saving performance of this device.

MOL Techno-Trade Co. Ltd. (MOLTech), agroup company of Mitsui O.S.K. Lines, Ltd.

2. Characteristics of the energy-saving(MOL), MOL, Japan Hamworthy Co., Ltd.

rudder with bulb fins(JHC), and Akishima Laboratories (Mitsui1) To recover the swirling flow of theZosen) Inc. (Akiken) have teamed up topropeller, the reaction shape is designed as aconduct research and development on ahigh thrust blade section and the rudder hornpropeller-mounted energy-saving deviceis twisted and integrated to increase thrust

called PBCF (Note 1) and the energy-saving

due to negative pressure on the front edge ofrudder with bulb fins.

the rudder.

The joint development of this innovative2) Decreasing the size of the end plate underrudder leverages the expertise of MOL,

the rudder reduces resistance and blocks

Akiken, and MOLTech, which have developed

propeller swirl flow.

and refined the PBCF for over 30 years, with3) The large central bulb has a flat front

the know-how of JHC, a specialized

surface to reduce propeller inflow speed andmanufacturer of high lift rudders, to improve

improve propeller efficiency.

not only ship maneuvering performance but
also horsepower with higher energy
efficiency while maintaining the steering

3. Overview of R&D
In our quest for the optimal energy-saving

effectiveness of the schilling rudder. With anrudder with bulb fins, we conducted

eye toward the autonomous vessels of theparameter studies as shown in Chapter 4

future as well as the potential applications onwith computational fluid dynamics (CFD)both oceangoing ships and coastal vessels,

calculation and a towing tank test, targeting a

we plan to begin sales of the energy-saving

Japan Bulk Carrier (JBC) hull, a generalrudder with bulb fins in 2021. While we areenlarged ship design that is widely used incurrently fine-tuning its performance, we will

various research and development, and anexplain the past results of our R&D efforts

Over Panamax car carrier as the fine ship.

below, as we obtained good results onFirst, we narrowed down the combination of(Note 1) PBCF: Propeller Boss Cap Fins. A device attached to the propeller of a vessel. It breaks up
the hub vortex generated behind the rotating propeller (resulting in energy savings of 3% to 5%compared to an identical vessel not equipped with PBCF).

optimal parameters by various CFDConventional rudder/schilling rudder

parameter studies and confirmed the results

With and without reaction of schillingby tank tests.

rudder, reaction angle

Generally, it is difficult to improve thehorsepower by 4% or more with a normal
reaction rudder. However, with this newly

the schilling rudder horndeveloped rudder, CFD showed a 5.7%
improvement in horsepower and a self-

The rudder has been applied to fine ships
as well, and CFD confirmed an energy-

Length of schilling rudder lower end
plate



propulsion test in a large towing tankconfirmed a 5.2% improvement.

With and without skeg in the rear ofWith and without flow regulatingplates in upper shilling rudder

Diameter of rudder bulbShape of rudder front edge

Fin shape (NACA symmetry/saving effect of 4.4%.

asymmetry blades)
Fin angle, length

4. CFD and water tank test
We will introduce some of the CFD

Fig. 1 shows pressure distribution in CFD

computation and tank test results as follows:on the rudder surface of the JBC hull. At left

1) CFD of a JBC hullis the CFD result of a conventional rudder,

We conducted various CFD computations

and at right is that of the energy-savingin combination with the following parameters

rudder with bulb fins. The pressure change

to narrow down the optimal energy-saving(red and blue color indicates positive andrudder with bulb fins.

negative pressure respectively) on the rudder(Examination parameters of the energy-

surface is caused by the rotation of thesaving rudder with bulb fins)propeller. In other words, it flows in from theport side at the upper rudder and from theWith and without PBCF

starboard side at the lower rudder. PressurePBCF cap shape (shell type/cylinder

resistance of the energy-saving rudder withtype)bulb fins is small, with outstanding negative
pressure on the front edge.

(Starboard side) Conventional rudder (Port Side)(Starboard side) Energy-saving rudder with bulb fins (Port Side)

Fig. 1. Pressure distribution on rudder surface of JBC hull in CFD(Red and blue color indicates positive and negative pressure respectively)

continued improvements in the estimating2) Tank test of JBC hull
Using the JBC hull, we conducted
resistance and self-propulsion tests andand verifying performance using CFD,
verified and compared with SPIV results at
the flow distribution level.

velocity measurement on stern by stereo
particle image velocimetry (SPIV) in a large
towing tank at Akiken. The various energysaving rudders with bulb fins including a
conventional rudder were narrowed down for
selection by CFD. The energy-saving effectof the newly designed rudder, as measured in
the tank test, almost matched the CFD result.

Fig. 3. SPIV measurement device

An example of the energy-saving rudder
with bulb fins is shown in Fig. 2.

Fig. 4. Photo during SPIV measurement

Fig. 2. Model of the energy-saving rudder with bulb fins

In addition, we conducted SPIV
measurement tests to understand themechanism of performance improvement andCFD verification. The SPIV measurement
device is shown in Fig. 3, the photo during
SPIV measurement in Fig. 4, and the result

Fig.5. Result of velocity measurement (the energy-of velocity measurement in Fig. 5. We cansaving rudders with bulb fins)expect more findings that will further improve
the rudder’s performance, as well as(Red and green color indicates high and low speed
range respectively)

3) Tank test using a JBC hull (PBCF unit

Fig.7 shows the flow speed distribution attest)

the propeller position in towing conditions

In the SPIV measurement of the PBCF

and pressure distribution on the rudder

unit shown in Fig. 6, you can see that thesurface. Red color indicates the range of highswirling flow behind the cap, which is seen inspeed/high pressure, and the range of blue

“without PBCF,” was eliminated and nocolor indicates low speed/low pressure.

longer evident in the “with PBCF” view.

Fig. 7. Flow speed distribution at the propeller(Without PBCF)(With PBCF)

Fig.6. Result of flow speed measurement (PBCF unit

position in towing conditions and pressure distribution onthe rudder surfacetest)(Red and blue color indicates high and low speed range
respectively.)

In addition, Fig.8 shows pressure
distribution on the rudder surface by CFD. At
left is the result of CFD for the normal rudder,

4) CFD using a car carrier
Based on findings gained by examination

and at right that for the energy-saving
rudders with bulb fins on the fine ship. In theof a JBC hull, we conducted the sameenergy-saving rudders with bulb fins, theparameter study about optimization by CFD

positive pressure range in the front of theof the energy-saving rudders with bulb fins on

rudder decreased and the negative pressure

the fine ship.

range increased. This contributes to higher
propulsive force.

(Starboard side) Conventional Rudder (Port side)(Starboard side) Energy-saving rudder with bulb fins (Port side)

Fig.8. Pressure distribution by CFD on the rudder surface on car carrier(Red and blue color indicates high and low pressure respectively.)5. Conclusion
As an upgrade of the schilling rudder, wedeveloped energy-saving rudders with bulb
fins that improve ship maneuvering andenergy-saving performance (improvinghorsepower) while maintaining steering
effectiveness. As we move toward installation
on in-service vessels, we will also confirm not
only the synergistic effect with PBCF, but also
the effect with ducts and pre swirl fins.
Meanwhile, we have acquired a patent on theenergy-saving rudders with bulb fins.
Finally, to our joint study partners—MOL,JHC, and Akiken, we truly thank you from thebottom of our heart.